The aim of the current proposal is to characterize a late sensitive period for the development of anxiety and mood disorders. Evidence to date suggests that disruption of the serotonergic system during post--&#8208;natal development in animal models results in altered anxiety and mood related behaviors in the full grown adult animal. In particular, there is evidence that proper signaling through 5--&#8208;HT1A receptors is required in the third postnatal week for the development of normal circuits that mediate anxiety. Thus, disruptions of occurring at this time likely result in the de novo formation of aberrant circuits. In the current proposal, we intend to test the hypothesis that a later sensitive period also exists after the initial circuitry has formed. In our model, once normal circuitry that sub--&#8208;serves anxiety and depression related behavior has formed, there is a vulnerable period during which time the circuits remain unstable and vulnerable to disruption. In this model, disruption of 5--&#8208;HT1A signaling at this time results in aberrant circuitry through aberrant maturation and stabilization of the circuits that have already formed. The experiments proposed are aimed at defining the exact window of late vulnerability. We will do this using a transgenic mouse approach that relies on reversible knockouts of the 5--&#8208;HT1A receptor. In addition, using reversible knockouts of 5--&#8208; HT1A auto and heteroreceptors, we intend to identify the specific population of receptors that is responsible for the late window.